How to thicken a patch

Create a bevel or endcap
Go to drop down menu labeled Curve and click on Thicken. shortcut is Ctrl+t (depending on which Radiant version you have, it works in 1.4 and 1.6)
The Amount equals the amount of units to thicken the curve, by default it is 8u. You can change it however u see fit,
but it looks best in increments of 8 (8,16,24 etc...).

Seams is pretty self-explanatory keeping the box checked will patch all other 5 sides with with a spacing of x amount of units.

Unchecking seams will duplicate original brush and invert it seperating it by x amount of units. This is good for using curves where the edges dont meet or are never seen by the player.

Texturing Thickened Curves

The proper way to thicken curves is to select the texture prior to thickening the curves. Doing it this way saves an extra step. Applying a texture after you have thickened the brush will end up looking like this.

To align the texture press Ctrl+n to smooth or Naturalize the texture so it doesn't look stretched.

Make sure to delete any unnecessary patches that wont be seen in game.

More on Caps

See Morbus' tutorial above on basic info for Cap selection. This tutorial is on the texturing process for it.
It's the same process as texturing thickened curves. Basically you want to select the texture before capping the curve.
Doing it this way will look like this...

Doing it the wrong way will look like this (applying the texture after you cap it).